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UGM 2000
Company Confidential Copyright 2000 Fluent Inc. All rights reserved.
1
A Turbulence Primer
A Turbulence Primer
UGM 2000
Company Confidential Copyright 2000 Fluent Inc. All rights reserved.
2
Outline
• Background
Scales
Eliminating the small scales
¾ Reynolds Averaging
¾ Near Wall Turbulence
¾ Filtered Equations
• Examples
Comparison with Experiments and DNS
¾ Turbulence Models
¾ Near Wall Treatments
UGM 2000
Company Confidential Copyright 2000 Fluent Inc. All rights reserved.
3
• Unsteady, irregular (aperiodic) motion in which
transported quantities (mass, momentum,
scalar species) fluctuate in time and space
• Fluid properties exhibit random variations
statistical averaging results in accountable,
turbulence related transport mechanisms.
• Contains a wide range of eddy sizes (scales)
typical identifiable swirling patterns
large eddies ‘carry’ small eddies.
What is Turbulence?
UGM 2000
Company Confidential Copyright 2000 Fluent Inc. All rights reserved.
4
Two Examples of Turbulence
Turbulent boundary layer on a flat plate
Turbulent boundary layer on a flat plate
Homogeneous, decaying, grid
Homogeneous, decaying, grid
-
-
generated turbulence
generated turbulence
UGM 2000
Company Confidential Copyright 2000 Fluent Inc. All rights reserved.
5
Energy Cascade
• Larger, higher-energy eddies, transfer energy to
smaller eddies via vortex stretching.
Larger eddies derive energy from mean flow.
Large eddy size and velocity on order of mean flow.
• Smallest eddies convert kinetic energy into thermal
energy via viscous dissipation.
Rate at which energy is dissipated is set by rate at which
they receive energy from the larger eddies at start of
cascade.
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